The invention relates to a transmission circuit for an electronic telephone set, comprising a receive circuit, a transmit circuit and a logic circuit, two of these circuits being connected as direct current series circuit.
Such a transmission circuit is disclosed in "The Proceedings of the 2nd International Symposium and Subscriber Loops and Services", May 3-7, 1976 pages 112 to 117, inclusive.
An electronic telephone set comprises a transmission circuit for transmitting information and for signalling purposes. The receiving circuit of the transmission circuit is used for the reception and amplification of the audio signals received by the telephone set through the subscriber line. The logic circuit of the transmission circuit, when dialing push-buttons are employed, is used to generate logic signals to obtain a frequency combination tone signal representing the chosen number.
The transmit circuit of the transmission circuit is used for amplifying and transmitting the audio signals which are conveyed to the exchange through the subscriber line. Generally, and particularly for long subscriber lines, it is desirable for the audio signal transmitted to the exchange by the transmit circuit to have the largest possible voltage amplitude so as to satisfy the requirement imposed by the regulatory authorities that the power of this audio signal must be above a minimum value set forth by the regulatory.
The receive circuit, the transmit circuit and the logic circuit all receive their supply direct current through the subscriber line to which the telephone set is connected. Each of the circuits can only function if its supply direct voltage exceeds a certain minimum value, which minimum supply voltage generally has a value which is different for each one of the circuits. The maximum supply current the subscriber line is capable of producing depends inter alia on the line impedance and consequently on the length of the subscriber line. It is possible that with the minimum supply voltage required by the circuits, a long subscriber line cannot produce the current required by the authorities.